Over 3,700 former National Football League (NFL) players have filed in dozens of federal lawsuits, including a class action involving over 2,000 former players, against bosses of the franchises, alleging that they have ignored evidence regarding the long term impact of head injuries received during their playing days.
They argue that such injuries have led to long-lasting debilitating health issues such as memory loss and headaches.
"The NFL refused to acknowledge that chronic brain damage in former NFL football players was an epidemic that constituted a national health crisis," a lawsuit filed by former Chicago Bears defensive back Shaun Gayle said.
The issue has come to the fore after Junior Seau, considered one of the greatest linebackers of all time, shot himself last May and died at the age of 43.
A post-mortem examination of his brain later revealed that like dozens of former players, he was suffering from the degenerative brain disease chronic traumatic encephalopathy.
Boston University's Center for the Study of Traumatic Encephalopathy, which analyzes the phenomenon, also found that former Chicago Bears' safety Dave Duerson had the same condition, which may have explained his depression and mood swings.
Duerson killed himself in February last year and donated his brain to the center.
Elsewhere, Cleveland Browns quarterback Colt McCoy played on following a head injury sustained in a game against the Pittsburgh Steelers last season.
And in the middle of the 2011 season, San Diego Chargers blocker Kris Dielman staggered after what appeared to be a normal play and after the game became disoriented before suffering a seizure on the flight home.
Experts determined later the career-ending incident came due to repeated collisions on the same play 15 times during the game.
Meanwhile, in November last year, San Francisco 49ers quarterback Alex Smith told coaches and team trainers he was suffering with blurred vision after a hit, leading to a concussion diagnosis.
And in the NHL, Pittsburgh star Sidney Crosby was sidelined for much of two seasons after serious head injuries, the biggest name in what had become a serious problem for the league.
Both sports have hit back at claims that they have not done enough to reduce the risks.
NFL-paid and union-approved trainers have spent the past year watching every play of every game in stadiums on monitors to better detect head injuries to players, saying any allegation of intentionally misleading players had "no merit".
The lawsuits stood "in contrast to the league's actions to better protect players and advance the science and medical understanding of the management and treatment of concussions," said NFL spokesman Brian McCarthy.
NFL commissioner Roger Goodell added that the league had been looking into concussions and financing head injury research for more than a decade and had spent nearly $100 million in in equipment-related safety research.
There has been a crack-down on blows to the helmet, using the helmet against opponents, greater medical caution for concussed players and those who might have concussions and changed kickoff rules to reduce injury odds.
For its part the NHL has cracked down on hits targeting the head, redesigned some padding and switched to a softer form of glass above the boards.
"These are all things that we're doing to try and react and be proactive because it's something that's serious," NHL commissioner Gary Bettman said.
The 49ers' Alex Smith said he has no regrets about speaking up, despite being benched and unable to regain his starting spot.
Goodell agreed: "There's nothing wrong with raising your hand and saying, 'I don't feel good.'"
But 14-year Denver receiver Brandon Stokley said he has played through concussions in the past and some players are still afraid.
"You always have that kind of feeling -- not necessarily that you're going to lose your job -- but guys still hide them. They will play through them," Stokley said.
"Everyone's awareness has changed. You realize there could be repercussions down the road."